The H ion concentration of the body fluids is kept within a narrow range primarily by the renal regulation of acid excretion. Acidosis, therefore, is a common feature of renal failure. The study of renal H ion transport is hampered by the complex structural organization of the kidney; hence, the use of simpler models such as the turtle urinary bladder. The advantage of this membrane is that it can be isolated in an environment where the driving forces are well-defined and the regulatory influences can be studied singly and in concert. This proposal plans to study H ion transport on three levels of organization: epithelial, cellular and subcellular. 1) Transport across epithelia can be described by two parallel paths, active and passive. We have devised a method that specifically measures flow in the active pathway. We will study the relation of the active and passive fluxes to the transepithelial gradients and to each other. The passive permeability of the membrane to protons will be changed, allowing us to formulate reasonable models for H ion transport in tight and leaky epithelia. 2) On a cellular level we will study the details of the coupling between transport and the various pathways of cellular respiration using newly devised methods that measure the oxidation of specifically labeled metabolic fuels. The energy cost of transport will be defined both in terms of fuel oxidation and utilization of intracellular high-energy intermediates. The effect of aldosterone and of chronic increases in transport work on the energy cost will be studied. 3) We will define the subcellular location of the proton pump. The pump is probably a membrane-bound proton-translocating ATPase. It will be isolated and its characteristics studied. Definition of the characteristics of active H ion transport in this epithelium should advance our information on the process underlying urinary acidification in health and in various disease states where this function is deranged, such as renal failure, hyperaldosteronism and some renal tubular diseases.